Thin-film rechargeable batteries have numerous applications in the field of microelectronics. For example, thin-film batteries may provide active or standby power for microelectronic devices and circuits. Active power source applications of thin-film batteries include, for example, implantable medical devices, remote sensors, semiconductor diagnostic wafers, automobile tire sensors, miniature transmitters, active radio frequency identification (RFID) tags, smart cards, and MEMS devices. Standby power source applications of thin-film batteries include non-volatile CMOS-SRAM memory products such as memory chips for computers, sensors, passive RFID tags, and backup power for real time clocks in electronic devices, for example, cellular telephones.
In a battery, a chemical reaction takes place between an anode and cathode by interaction of the anode and cathode through an electrolyte that may be a solid or liquid. Liquid organic electrolytes used in conventional lithium-ion batteries pose safety problems because the electrolytes are flammable and are not tolerant to temperatures above about 130° C. The attractiveness of thin-film, solid state batteries over conventional batteries is that the electrolyte is a solid or non-flowable material rather than a liquid. Solid state, thin-film batteries typically employ glassy ceramic electrolytes. Solid electrolytes are desirable in cells or batteries where liquid electrolytes may be undesirable, such as in implantable medical devices. Preferred solid electrolytes include materials that are amorphous solids with high melting temperatures (greater than about 900° C.), and are electrically insulative and ionically conductive.
The demand to reduce power consumption of electronics is one of the drivers to lower the operating voltages of integrated circuit (IC) components such as processors and non-volatile memory. Presently many components operate at about 2 volts and below. Rechargeable lithium or lithium-ion batteries with oxide based cathodes such as LiCoO2 and metallic lithium, lithium alloy, or carbon anodes typically have a voltage output between 4.2 volts and 3.0 volts. While present thin film lithium-ion batteries can be discharged to zero volts, they essentially provide no operating capacity below about 3.0 volts.
In order to power low-voltage electronics with a conventional thin film lithium ion battery, a voltage regulator circuit is required which wastes battery energy, and occupies additional space, and adds to the heat load of the circuit. For millimeter-sized and smaller devices, this defeats the purpose of using a thin-film micro-battery. The purpose of the disclosure is to provide a thin film battery that delivers most of its capacity between 2.5 volt and 0.5 volts.
With regard to the above, an exemplary embodiment of the disclosure provides an apparatus for use as a low-voltage, thin film battery and methods for making a low voltage battery. The apparatus includes a ceramic substrate having a first surface and a second surface. A cathode current collector is disposed adjacent to at least a portion of the first surface of the substrate. The current collector has a titanium layer and a gold layer. A high temperature annealed cathode is disposed adjacent to at least a portion of the gold cathode current collector and an electrolyte layer disposed adjacent to the annealed cathode. An annealed anode is disposed adjacent to at least a portion of the electrolyte, wherein the anode is electrically insulated from the cathode and the cathode current collectors by the electrolyte. An anode current collector is disposed adjacent to at least a portion of the anode.
Another exemplary embodiment of the disclosure relates to a method for making low-voltage, thin film battery. The method includes depositing a cathode current collector on at least a portion of a first surface of a substrate. The cathode current collector has a titanium layer adjacent to the substrate and a gold layer adjacent to the titanium layer. A cathode film is deposited on at least a portion of the gold film. The cathode film is then annealed in air at a temperature ranging from 750° C. to 800° C. for a period of time ranging from 10 minutes to 30 minutes. An electrolyte layer is then deposited on at least a portion of the annealed cathode. An anode film is deposited on at least a portion of the electrolyte layer, and an anode current collector is deposited on at least a portion of the anode film.
As disclosed herein, the thin film battery has an output voltage between less than 3.0 volts and about 0.0 volts, and typically delivers most of its voltage potential between 2.5 and 0.5 volts. Accordingly, the thin film battery, as described herein, may be used in low voltage circuits without requiring the use of a voltage regulator. Also, the thin film battery, as described herein may be cycled at temperatures above about 100° C. and relatively high currents. For example, the battery may be able to withstand temperatures in excess of about 265° C. thereby enabling assembly of integrated circuits containing the battery using ROHS-compatible solder reflow or surface mount assembly techniques.